Conventionally, as a backup function provided in the storage system, the copy function of creating the mirrors of logical volumes in the same storage system without passing through the host (hereinafter referred to as the “in-storage copy function”) exists.
Meanwhile, as another backup function provided in the storage system, the copy function of creating the mirrors of logical volumes between two storage systems (hereinafter referred to as the “remote copy function”) is also well-known.
As the methods for remote copy using the remote copy function, the synchronous method by which remote copy to logical volumes as the backup destination is performed for data in synchronization with the data written to logical volumes and the asynchronous method by which remote copy to logical volumes as the backup destination is performed for data asynchronously with the data written to the logical volumes as the backup target exist.
In the synchronous method, after the data written to the logical volumes is copied to the logical volumes as the backup destination, a remote copy completion report is transmitted to the host. Therefore, the synchronous method has the problem that, though backup is highly reliable, if the distance between the two storage systems is large, remote copy takes a fair amount of time, which degrades the copy efficiency.
Meanwhile, the asynchronous method makes a remote copy completion report to the host when the data has been written to the logical volumes, and then remote copy is performed for the relevant data at the convenience of the storage systems of the copy source and the copy destination.
Therefore, the asynchronous method has the problem that, though copy can be performed efficiently even if the distance between the two storage systems is large, backup has lower reliability.
In view of that, the remote copy method combining the synchronous method and the asynchronous method for solving the respective problems has been proposed. This method locates the synchronous remote copy destination site relatively close to the remote copy source site, and locates the asynchronous remote copy destination site farther than the synchronous remote copy destination site.
This method is highly fault tolerant as backups exist in the synchronous remote copy destination site and in the asynchronous remote copy destination site respectively, and is known as the multi-target type remote copy.